The invention relates to a vortex flow measuring instrument in the manner of so-called vortex flow meters.
The invention relates here as an example to the known measurement principle which is employed in vortex flow meters (VFMs).
The invention is based on vortex flow meters (vortex meters) which are nowadays used to determine volume flow or flow rate. Vortex flow meters can measure the flow of gases, steam and liquids in a wide measurement range, independently of the substance properties. The function of the vortex flow meter is based on the Karman vortex street. At the bluff body past which the medium flows, reciprocal vortices are formed on both sides. Owing to the flow, the vortices are shed and a vortex street (Karman vortex street) is formed.
The frequency f of the vortex shedding is proportional to the flow rate v and indirectly proportional to the width of the bluff body d:f=St*v/d
The Strouhal number, denoted as St, is a dimensionless parameter which critically determines the quality of the vortex flow measurement. With suitable dimensioning of the bluff body, St behaves constantly over a very wide range of the Reynolds number ReRe=v*D/υ    υ=kinematic viscosity    v=flow rate    D=inner diameter of the measurement tube    St=Strouhal number
The vortex shedding frequency to be evaluated is consequently now dependent only on the flow rate and independent of the density of the substance to be measured and the viscosity. The local pressure changes involved in the vortex shedding are usually detected by a piezo sensor and converted into electrical pulses corresponding to the vortex frequency. This frequency signal is processed further and conditioned in the measuring transducer.
The measurement signal of the vortex meter is the frequency of the vortices shedding from a flow obstacle. This frequency is proportional to the flow over wide ranges of the flow.
Depending on the application, the components of the vortex flow meter are made of various materials or combinations (for example steel, chromium-nickel steels, Hastelloy, titanium or tantalum). In each case it is necessary to ensure that these materials, which come in contact with the medium to be measured, are resistant to the substance to be measured.